Question

Question #71b91

Answer 1

I don't quite understand what the question means so I'll show how to work out `A_r` from these data if that helps.

A good way is to imagine that you have 100 atoms of the element.

Out of this 100 atoms you will have 37.4 atoms of Re 185 and 62.6 atoms of Re 187. (Ok you can't have fractions of atoms but this is to make the sum easier to explain).

So now work out the mass that each isotope contributes. This will be its mass x the no. of atoms present.

So the mass of all the Re 185 atoms in 100 atoms of the element will be:

37.4 x 185 = 6919 amu

And the mass of all the Re 187 atoms will be:

62.6 x 187 = 11706.2 amu

So the total mass of both isotopes will be 6919 + 11706.2 = 18625.2 amu

So the total mass of 100 atoms will be 6919 + 11706.2 = 18625.2

So the average mass of 1 atom is 18625.2/100 = 186.252 amu.

This gives the `A_r` of Re.

So I guess the sum is twice the average mass =186.252 x 2 = 372.504 amu